Conventionally, it has been known that, when an electromagnetic wave is irradiated to a living body, an acoustic wave is generated due to a temperature increase and thermal expansion of tissue of the living body caused by absorption of the electromagnetic wave by the living body. A technique (referred to as photoacoustic tomography (PAT)) for visualizing an inner portion of the living body in a non-invasive manner by using this phenomenon has been attracting attention and has been employed in clinical sites using photoacoustic tomography diagnostic apparatuses.
In a photoacoustic tomography diagnostic apparatus, a specimen as a target is irradiated with light, and an acoustic wave generated thereby is received by a one-dimensional or two-dimensional micro-transducer array in which a plurality of micro-transducers are arrayed. As the one-dimensional or two-dimensional micro-transducer array, probes of a kind used for an ultrasonic diagnostic apparatus generally are used.
For image reconstruction in the photoacoustic tomography, various algorithms are applied. In general, a delay-and-sum process used for the image reconstruction in the ultrasonic diagnostic apparatus may be adapted.
After the light irradiation of the specimen, although the acoustic wave generated from the target position is received during the time of receiving the acoustic wave, the distance from the target position to each of the micro-transducers is not the same (equal) for each of the micro-transducers. For this reason, the acoustic wave signal generated from the target position reaches the micro-transducers at different time points. Therefore, in general, in the photoacoustic tomography diagnostic apparatus, the time differences in the acoustic wave signals that arrive at the detectors at different time points are adjusted for by using the delay-and-sum process so as to generate the photoacoustic tomography image data corresponding to the target position. The generated data of the target position are minimum constitution units (called “pixels” or “voxels”) of the two-dimensional or three-dimensional photoacoustic tomography image. In the delay-and-sum process, the acoustic wave analog signals received by the micro-transducer array are amplified by an amplifier and converted to digital signals by A/D converters, and these digital signals are stored in a storage device. Accordingly, signal values that originated from the same target position are added for all required channels.
In addition, in the photoacoustic tomography diagnostic apparatus, a process called apodization is performed in order to improve directionality of the one-dimensional or two-dimensional micro-transducer array. This process is, instead of uniformly adding the acoustic wave signals received from the micro-transducers in the micro-transducer array, rather, attenuating the acoustic wave signals that reach a region of the micro-transducer array. This process improves the directionality of the micro-transducer array by suppressing the strength of the acoustic wave signals originated in directions other than the target direction. In general, different weighting factors are applied to the acoustic wave signals received by the micro-transducers so that the same effect as applying the window functions or functions depending on solid angles and distances to the acoustic wave signals can be obtained.
In the delay-and-sum process on the digital signals, a delay apparatus for adjusting the delay times for the receiving channels is used. As the delay apparatus, a storage device such as a first in first out (FIFO) memory or a RAM is mainly used.
Recently, a large scale of a field programmable gate array (FPGA) chip has been provided. Moreover, high-speed rewritable FIFO memories or RAM memories are mounted thereon. Therefore, the FPGA chip can be easily mounted on the received data processing apparatus of photoacoustic tomography. However, the high-speed logic memories mounted on the FPGA chip have a limitation in terms of memory capacity. In addition, since large-scale FPGA chips are expensive, the received data processing apparatus of photoacoustic tomography needs to be configured with as small a logic memory capacity as possible.
Japanese Patent Application Laid-Open (JP-A) No. 2005-21380 or Japanese Patent Application National Publication (Laid-Open) No. 2001-507952 may be referred to for discussion of technologies of irradiating a specimen with light, receiving an acoustic wave generated due to thermal expansion of the specimen caused by the light irradiation, and constructing an image based on electrical signals obtained from the acoustic wave.